HAS-A relationship is based on usage, rather than inheritance. In other words, class A has-a relationship with class B, if code in class A has a reference to an instance of class B.
example
class Student
{
String name;
Address ad;
}
here we say that student has-a address
Java Aggregation (HAS-A relationship)
原文：http://www.cnblogs.com/morningdew/p/5618879.html